Induction well logging system



Aug. 22, 1961 R. H. HUDDLESTON, JR, ETAL INDUCTION WELL LOGGING SYSTEM Filed Nov. 15. 1957 4 Sheets-Sheet l FIG. I.

LOGGING TOO INDU D CURR T BOREHOLE l2 iL mnucso CURRENT SOURCE COIL I/ INVENTORS. RICHARD H.HUDDLESTON JR.. MELVIN acnuu,

AGEN

4 Sheets-Sheet 2 FORMATION WELL mums moucso CURRENT mouczn CURRENT INVENTORS, RICH H HUDDLESTON JR MEL E. CHUN,

AGENT.

INDUCTION WELL LOGGING SYSTEM Aug. 22, 1961 R. H. HUDDLESTON, JR., ET AL Filed Nov. 15. 1957 Ia:E E, Az

RECEIVER COIL SOURCE COIL l4 1961 R. H. HUDDLESTON, JR, ET AL 2,997,645

INDUCTION WELL LOGGING SYSTEM 4 Sheets-Sheet 3 Filed Nov. 15. 1957 INDUCED CURRENT INDUCED CURRENT INVENTORS. RICHARD H- HUDDLESTON JR.

MELVIN E. CHUN, BY W 972.

AGENT.

Aug. 22, 1961 Filed Nov. 15. 1957 INDUCTION WELL LOGGING SYSTEM ISA FIG. 7.

4 Sheets-Sheet 4 22k MATCHING SECTION 7 DETECTOR CIRCUIT MODULATOR 38 3? DETECTOR cmcurr MODULATOR 3O 36 DETECTOR CIRCUIT MODULATOR 2O I ZOK-C.

GENERATOR coll. l4

SPOOL 44 INVENTORS.

RCHARD H.HUDDLESTON JR. MELV|N E. CHUN,

A GENT This invention generally relates to apparatus for logging well formations and more particularly relates to induction well logging apparatus.

Induction logging apparatus presently in use basically includes a flux field source coil, energized by alternating current, disposed in separately spaced relation with one or more receiver coils, the spaced coil array .being a'dapted tofb lowere through a well here. The d p' d iqe b h s me 1 'd f e d were in the surround pg earthforr'nations and well bore fluids in paths concentric w :11; the axis of the con; Such enn w e no e a am n. m e c fie d-t range are produced in th e receiver coil by both the directly p Q fl1I ;-fi' f h f a fly p c u e fll field. some phasedetec on means is provided to separate that voltaige H'C inponent inducedijn the receive'r coil y th s o ar or Le dy een e ate fi .f qm that directly inducedby the source coil, this secondarily u d m 9m bei tindi a t Qf l. ductivity of media surronn'ding the source-receiver coil y- The conductivity of the wen V a large part of thejeddy current paths and t th s fi fl 'ity e l tiq w t h effort to. reduc or obviate effects, the legging tool has been por other insulating extending M... k whi s h r 1 been practical G Q me extent, the ifi'ns hav mad ficult and sometimes impossible to pass r ea-s61 through the well bore. Also, such fins have neefiecte the conductive mud filtrate inva ded one of formation ifnmediately surrounding the well bore. o v

Another arrangement for reducing the undesirable bore hole efieet been to provide one or more auxiliary coils respectively connected in series: opposition with either the source cOiLreceiVer coil, 'or both, to reduce the sensitivity of the coil array in its immediate vicinity. While this system may accomplish such purpose, it has been found when calibrated fora null output in a media of particular conductivity or a well bore filled with of particular conductivity, that sharp contrast in conductivity between the well bore fluids and thesurrounding formations will lead to spurious indications. For example, when traversing a well bore filled with fluid of high conductivity and a surrounding formation stratum of high conductivity, the system could be calibrated to produce an accurate indication of the formation conductivity. However, when "next traversing a stratum of low conductivity, the high conductivity of the well Here fluids could actually produce a negative response, indicating negative conductivity. in H An early reference to neuronal 'iiiult'ipleco' tagging systems and d scmsnre or the (slab :ed res onse as coils connected in series oppbsitiofi to reduce the dii" duced voltage and the bore hole elfect is Patent No. 2 9 0 t Aik m t luth il n Q.f., Yd Q containing founations invaded by mud filtrate, it is thought that .hanked up" connate water is displaced ahead of the mud filtrate into a 'coneentriczone betweenthe invading filtrate and the hydrocarbons. Multiple electrode logging tools have been devised to detect this high conductivity zone. However, if logged by present induction systems, this zone will tend to become crowded by eddy current lines of the induction logging source coil and obscure the relative contribution to the total current conduction by other portions of the formation, making, for instance, the contribution of the virgin formation less significant and detracting from the value of the induction log as a conductivity reading device. For example, a hydrocarbon containing formation having an actually low conducjtivity may be indicated as a high apparent conductivity of a Water containing formation.

It is therefore the general object of the present invention to provide a new induction logging system sensitive to changes in conductivity of the virgin formation remote from the well bore and insensitive to the conductivity of well bore fluids and immediately surrounding filtrate invaded zones.

It is a further object of the invention to provide a new. well logging apparatus requiring simple phase insensitive detection apparatus.

These and other objects of the invention are attained by induction well logging apparatus, including, a flux field source coil disposed in spaced apart relation along the axis of the tool from at least one receiver coil, the axis of the source coil being disposed parallel to the axis of the receiver coiland disposed at obliqueangle to the axi a the tool to ca se uiel fiux tag directly produced by the source coiland'the flux field secondarily produced by induced eddy currents immediately adjacent to the source coil to induce noyoltage in the receiver coil, and to cause the flux field secondarily pr'oduced by induced eddy currents more remote from the source coil to induce a voltage in the receiverncoi l, the spacing between the source coil and the receiver coil being sufliciently great to neglect the dimensions efiect of the coils.

Other objects and advantages of the invention will become more" apparent from the following description and claims taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a partly sectional elevation of a portion of the logging apparatus as immersed in ;a well bo're.

FIGURE 2, is a partly sectional elevation 'of the logging tool taken at from FIGURE 1. y

FIGURE 3 is a section taken at 33 of FIGURE 1.

FIGURE 4 is a section taken at 44 of FIGURE 1.

FIGURE 5 is a section taken at 5-5 of FIGURE 1.

FIGURE 6 is a schematic illustration of a detection and transmission system of thelogging fo'bl.

FIGURE 7 is a longitudinal cross-section taken at 7-7 of FIGURE 2 illustrating: a typical coil construction of the present invention in greater detail.

It flpyv ref rin to 1, t lsrqi d s o i P b or mandrel portion of a logging tool 10 disposed in a well bore '12. Tool 10 includes a flu id tight housing to contain the circuit elements shown in FIGURE 6 and is suspended in the well bore from a single conductor electrical logging cable (not shown). A flux field source coil 14 is disposed in the tool in separately spaced relation to one or more receiver coils 16A, 16B and 16C. The axis of source coil 14 is parallel to the axes of the receiver coils 16 and at an oblique angle to the axis of tool 10. The centers of all the coils 14 and 16 are aligned on the axis of tool 10.

When the source coil %14 is provided with an alternating exciting current, a flux field is produced in the surrounding earth formation generally illustrated by the dashed lines. Eddy currents, concentric with the axis of the source coil 14, are induced in the surrounding formation. Such induced current, random paths of which are designated at 18 in FIGURES l and 2, are also seen to be at an oblique angle to the axis of tool 10 and well bore 12.

FIGURES 3, 4 and 5, which are sections taken across the axis of source coil 14, illustrate the concentric paths of the eddy current in the -well formation relative to source coil 14. As seen in FIGURES 3, 4 and 5, eddy currents 18 follow paths concentric to the source coil 14 which, when not immediately adjacent to coil 14, are totally, or for the most part, in the well formation rather than the well bore 12. Thus, the eddy currents 18 defining paths entirely or largely in part in the Well bore fluids, will be confined to the immediate vicinity of the source coil 14. It is further seen that a secondary flux field produced by the eddy currents flowing totally or largely within the well bore would be of substantially the same configuration and disposition as the directly produced flux field illustrated in FIGURE 1.

As shown by the flux field lines of FIGURE 1, the source coil 14 and receiver coils 16, when disposed with axis parallel and separated by a distance which is large compared to the dimensions of each coil, will have a zero relative inductive coupling if the angle between the axes of the coils and the axis of the tool is such that the flux lines cut the turns of the receiver coils 16 perpendicular to the axis of the coil.

Proof of the principle of zero coupling between two or more coils so disposed, based on theory of the potential, is as follows.

Where The magnetic field at a given instant, set up by the source coil, will be the same at a distance from the coil as that produced by two equal and opposite poles |m and m, separated by a short distance 2l. This field is indicated by the dashed lines. Assume that receiver coil 16A, 16B and 16C are placed with axes parallel to source coil 14. The changing flux set up by source coil 14 will not induce any voltage in receiver coil 16A if the magnetic flux cuts the receiver coil at right angles.

The potential due to the pole +m at a point P will be that due to the pole m will be by the following equation the total potential at P will be i rl cos 6 if I is small, 1 may be neglected in comparison with r. Therefore,

2 Z ye -wee y 1=0 the equation of the locus is, therefore,

The locus is, therefore, a straight line at an angle of 54.73 with the axis of source coil 14. This line is shown in FIGURE 1 as the axis of tool -10. Within the limits of the simplifying assumptions made, receiver coils whose axes are parallel to the axis of source coil 14 and which are centered on this locus will have no coupling to source coil 14. Since the locus is a straight line, a series of receiver coils 16 having axes parallel to each other and centered on this locus will have no coupling to each other.

The oblique angle, thus theoretically found to be 54.73", is found in actual practice by model studies with coils of predetermined dimension and relative spacing in a surrounding medium of known conductivity.

Optimum conditions may be found at angles deviating slightly from the theoretical.

Though the apparatus is disclosed with a plurality of receiver coils 16, a single receiver coil properly spaced from the transmitter coil 14 would have a voltage induced therein indicative of the surrounding formation remote from the conductive effects of the well bore and immediately surrounding invaded formation. Thus the basic logging tool provided by this invention could have a single source coil 14 and a single receiver coil 16 spaced and disposed as previously described.

When provided with a plurality of receiver coils 16,

as prevently disclosed, and by choosing the proper number of turns for each receiver coil, the sensitivity for each spacing may be provided equal for a homogeneous medium such that the output from each receiver coil could be used for independent indication of a greater zone of investigation of the surrounding formation. For example, receiver coil 16A could be spaced sufficiently close to the source coil 14, relative to the coil dimensions, to provide an indication influenced in part by eddy currents flowing in the well bore fluids and invaded zones in the surrounding vicinity of the source coil. The receiver coil 16B, spaced at a farther distance from source coil 14, would not be influenced by the eddy currents close to the source coil but would give some indication of the possible concentric connate water bank zone of greater conductivity previously mentioned as occasionally found in invaded hydrocarbon bearing zones.

The conditions for zero coupling in the present apjparatus are such that when the diameters of the differential current loops become larger with respect to the spam .ing between source coil 14 and receiver coil 16, the principle of zero coupling no linger holds and coupling with the receiver coils readily occurs.

It is seen that increase in the induced current loop diameter causes increased coupling with the receiver coils 16, but, as shown in FIGURES 3, 4, and 5, such increase in diameter or increase in axial spacing from receiver coil 14 is provided through paths only partly intercepting the bore hole fluids or totally removed from the bore hole fluids. The effective coupling thus occurs in response to current flow in the zones least efiected by well bore fluids. 1

Since the induced current loops are about an axis oblique with the axis of the tool 10, they will tend to cross horizontal interfaces between beds of varied conductivity. While this action is contrary to the effect produced by present induction logging systems, certain advantages, in addition to the reduced bore hole effect, are thought to result. Such an advantage is that the apparent thickness of conductive beds can be reduced to a more nearly correct indication as compared with present systems. The effect of thin resistive beds will be more pronounced. The conductive drilling fluids within the well bore and/or the concentric filtrate invaded zone of high conductivity found in hydrocarbon bearing formations become far less influential as zones of maximum current density and enable the present apparatus to secure more accurate indications of true formation conductivity.

A schematic detection and transmission circuit for the present logging tool is illustrated in FIGURE 6. As shown, the source coil 14 is supplied with a source of constant alternating current from a generator means 20. Generator 20 is powered through the logging cable and a matching section 22 from the earths surface with a low frequency AC. power source, for example 400 cycles. The frequency of generator 20, herein exampled as 20 kilocycles, may be other selected frequencies with equal utility to the operation of tool 10. In operation, generator 20 operates continually to supply a constant current through the source coil 14 to effect a constant flux field in the environment of coil 14.

The receiver coils 16A, 16B and 16C are respectively connected through amplifiers 24, 26 and 28 into detector circuits 30, 32 and 34, each of which produce a DC. for example, outputpotential directly proportional to voltages induced in the receiver coils. The DC. output from each detector is fed into a modulator section of each of frequency modulators 36, 38 and 40. Such modulator sections convert the DC. potential into an AC. modulating voltage, for example 200 cycles, which modulates the frequency modulated oscillators from a preselected center carrier frequency at 200 cycle rate. Center frequencies of FM frequency modulated transmitters 36, 38 and 40 may be widely varied with equal utility to the operation of this tool. As herein exampled, such transmitters may be at frequencies of 10.5 kilocycles, l4 kilocycles and 22 kilocycles. The modulated frequency output from transmitters 36, 38 and 40 is fed through the matching section 22 onto the conductor of a well logging cable and thence to the earths surface to demodulating and discriminating equipment for subsequent indication and recording.

For further clear description of such frequency modulated well logging transmission systems reference may be had to Patent No. 2,573,133 to Greer.

A typical coil of the present invention and a manner of constructing the tool is shown in FIGURE 7. As shown, an inner mandrel 42, of fiber glass or plastic, extends throughout the induction coil section of tool 10. Disposed on the mandrel 42 are the coils wound in spools 44. 'As shown, the inner bore of the spools 44 is at the angle oblique to the coil axis as previously described. For directional orientation of the spool 44 with the inner mandrel 42 a keyway (not shown) may be provided along the mandrel and within the spool '42 for mutual registry when properly oriented. Extending from the mandrel to the outer periphery of the spool 44 is a body 46 which may be fiber glass, plastic or other such insulating and non magnetic material. An outer sheath 48 is disposed about the spool 44 and body 46 to provide a fluid tight exterior for the entire coil section of tool 10. The entire section is then bonded together into an integral rigid probe member.

The coil shown in FIGURE 7 may be either the source :coil 14 or any of the receiver coils 16, the only variation being the number of turns provided in each coil.

In operation, the receiver coils 16A, 16B and may be .of such coil turns and spacing to provide equal sensitivity in a homogeneous medium. Then, when the tool 10 is inserted in a well bore, the receiver coil of longest spacing, such aslcoil 16C, would have a voltage induced therein responsive only to the conductivity of formation removed from the well bore. Receiver coil 1613 being of lohg but closer spacing from source coil 14, would also indicate the conductivity of formation remote from the well bore, but, due to the decreased spacing, would tend to indicate interfaces of formations of different conductivity between coils 146C and 16B. The coil 16A may be spaced from source coil 14 at such spacing that the induced voltage may be partially responsive to the eddy currents of the well borefluids or filtrate invaded zones immediately surrounding the well bore. Thus, a correlation of the record of coil 16A with those of coils 16B and 16C would give an indication of the extent of the filtrate invaded zone and possibly the presence of the concentric conna-te water bank previously described.

It is understood however, that the basic invention herein disclosed requires but a single source coil 14 and a single properly spaced receiver coil 16.

Thus, the system herein described provides an induction logging system capable of detecting the conductivity of well formations without the detraction of the well bore fluid and filtrate invaded zones eddy currents though the presence and relative magnitude of such invaded zones may be indicated by the apparatus if desired. Further, by proper spacing of a plurality of the receiver coils 16, interfaces between formations of different conductivity may be sharply detected.

It is seen that other arrangements and modifications of the present apparatus may be made as apparent to those skilled in the art without departing from the spirit of the invention or the scope of the following claims:

That being claimed is:

1. An induction well logging apparatus adapted to be lowered into investigating relationship within a well formation, including, a flux field source coil disposed in spaced apart relation from a receiver coil with the axis of the source coil in substantially parallel relation with the axis of the receiver coil and in oblique relation to ,a line defined between centers of the coils at an optimum angle for causing the lines of the flux field directly produced bythe source coil and the lines of a flux field secondarily produced by induced eddy currents in conduotive media immediately about the source coil to induce negligible voltage in the receiver coil while causing the lines of a flux field secondarily produced by induced eddy cur-rents in conductive media remote from the source coil to induce a voltage in the receiver .coil proportional to said remote eddy currents, there being suificient spacing between the coils to consider the source coil 21 point source of magnetic flux field and the receiver coil a separa-te point of induced voltage reception.

2. The apparatus of claim 1 including a constant current generator in connection with the flux field source coil to produce .a constant electromagnetic fiux field in surrounding well formation, a detector in connection with the receiver coil to detect the induced voltage, and modulated transmission means of selected carrier frequency in connection with the detector to transmit a signal representative of the induced voltage to the earths surface, the induced voltage being in responsive variation to the conductivity of said remote media.

3; Aninduction well logging apparatus to be lowered into investigating relationship within a well formation,

including, a flux field source coil' disposed in spaced apart relation from a plurality of receiver coils with the axis of the source coil in substantially parallel relation 'with the axes of the receiver coils and in oblique relation to a line defined between centers of the coils at an angle adequate for causing the lines of the flux field directly produced by the source coil and the lines of a flux field secondarily produced by induced eddy current in conductive media immediately about the source coil to induce no voltage in the receiver coils while causing the lines of a flux field secondarily produced by induced eddy current in conductive media more remote from the source 'coil to induce a voltage in the receiver coils proportional to said remote eddy currents, there being sufiicient spacing between the coils to consider the source coil a point source of magnetic flux field and the receiver coils points of reception of induced voltage.

4. The apparatus of claim 3 including a constant current generator in connection with the flux field source coil to produce a constant electromagnetic flux field in surrounding well formation, a detector in connection with each receiver coil to detect the induced voltage, and transmission means in connection with each detector to transmit a signal representative or its induced voltage to the earths surface, the induced voltage being in pro portionate variation to the true conductivity of the surrounding media.

5. An induction well logging apparatus adapted to be lowered into investigating relationship within a well formation, including, a flux field source coil disposed in spaced apart relation from a receiver coil with the axis of the source coil in substantially parallel relation with the axis of the receiver coil and in oblique relation to a line defined between centers of the coils at an angle sufiicient for causing the lines of the flux field directly produced by the source coil and the lines of a flux field secondarily produced by induced eddy currents in conductive media immediately about the source coil to induce negligible voltage in the receiver coil while causing the lines of a flux field secondarily produced by induced eddy currents in conductive media remote from the source coil induce a voltage inthe receiver coil proportional to said eddy currents, said angle being substantially 54.73 and there being sufiioient spacing between the coils to consider the source coil a point source of magnetic flux field and the receiver coil a point induced voltage reception.

6. An induction well logging apparatus adapted to be lowered into investigating relationship within a well formation, including, a flux field source coil disposed in spaced apart relation from a receiver coil with the axis of the source coil in parallel relation with the axis of the receiver coil and in oblique relation to a line defined between centers of the coils at an angle sufficient for causing the lines of the flux field directly produced by the source coil and the lines of a flux field secondarily produced by induced eddy currents in conductive media immediately about the source coil to induce negligible voltage in the receiver coil while causing the lines of a flux field secondarily produced by induced eddy currents in conductive media more remote from the source coil to induce a voltage in the receiver coil proportional to said more remote eddy currents, the tangent of said angle being approximately equal to /2. and there being sufficient spacing between the coils to consider the source coil a point source of magnetic flux field and the receiver coil a point source of reception of induced voltage.

7. An electromagnetic well logging tool adapted to be lowered into investigating relationship within a well tormation, including, a flux field source coil disposed in spaced apart relation along the axis of the tool from each of a plurality of receiver coils, the axis of the source coil being disposed substantially parallel to the axis of 8 each receiver coil and disposed at such an angle to the axis of the tool as' to cause the flux field directly produced by the source coil and the flux field secondarily produced by induced eddy current disposed immediately adjacentto and concentric with the source coil to induce negligible voltage in the receiver coils, and to cause the flux field secondarily produced by induced eddy current more remote from the source coil to induce voltages in the receiver coils, the spacing between the source coil and each receiver coil being sufliciently great to neglect the dimensions effect of the coils.

8. An apparatus for investigating the earth formations traversed by a well bore, including, an elongated probe member adapted to be lowered in aligned relation through a well bore, a constant electromagnetic flux field source coil means mounted with said probe for producing eddy current in conductive material surrounding said probe which flows about an axis intersecting said probe and defining an oblique angle with the axis of said probe, and induced voltage receiver coil means mounted with said probe at a distance from said source means large in relation to the dimensions of said coil means for producing no voltage directly induced by lines of said source means flux or induced by lines of flux secondarily produced by eddy currents in close vicinity of said source means while producing a voltage induced by lines of flux secondarily produced by eddy currents more remote from said source means, said induced voltage being indicative of the conductivity of the material more remote from said source means. 9. The apparatus of claim 8 including a detector in connection with said receiver coil means to detect the induced voltage, and transmission means in connection with said detector to transmit a signal representative of said induced voltage to the earths surface, and means to indicate said transmitted signal as a function of material conductivity.

10. An apparatus for investigating the earth formations traversed by a well bore, including, an elongated probe member adapted to be lowered in aligned relation through a well bore, a constant electromagnetic flux field source coil means mounted with said probe for producing eddy current in conductive material surrounding said probe which fiows about an axis intersecting said probe and defining an oblique angle with the axis of said probe, the tangent of said angle being approximately equal to /2, and induced voltage receiver coil means mounted with said probe at a distance from said source coil means large in relation to the dimensions of said coil means for producing negligible voltage directly induced by lines of said source means flux or induced by lines of flux secondarily produced by eddy current by close vicinity of said source means while producing a voltage induced by lines of flux secondarily produced by eddy current more remote from said source means, said induced volt-age being indicative of the conductivity of the material more remote from said source means.

11. In apparatus for investigating earth formations traversed by a well bore, an elongated probe member adapted to be lowered in aligned relation through a well bore, an electromagnetic flux field source coil means mounted with said probe for producing eddy current in conductive materials surrounding said probe which flows about an axis intersecting said probe and defining an oblique angle with the axis of said probe, said angle being approximately 55, a first induced voltage receiver coil mounted with said probe with its axis substantially parallel with said source coil and at a sufiicient distance from said source coil for producing negligible voltage directly induced by said source coil flux field and a discernible voltage induced by flux secondarily induced by eddy currents in close vicinity of said source coil in addition to any voltage induced by flux secondarily induced by eddy current more remote from said source coil which discernible voltage is indicative of the conductivity of the 9 material in close vicinity of said source coil, and a sec- References Cited in the file of this patent 0nd induced voltage receiver coil means mountedhwiltlh UNITED STATES PATENTS said robe with its axis substantiall arallel Wit t e axis 2f said source coil and at sufiigie i it distance from 1129584 Murphy 1915 said source coil for producing negligible voltage directly 5 1896737 Zuschlag 1933 induced by said source coil flux or flux secondarily pro- 2338793 Zuschlag 1944 duced by eddy currents in close vicinity of said source FOREIGN PATENTS coil While producing a voltage induced by flux secondarily 574,808 Great Bn'tain Jan 22, 1946 produced by eddy current more remote from said source coil which voltage is indicative of the conductivity of 10 the material more remote from said source coil. 

